I. Field of the Invention
The present invention is directed generally to a packaged, temperature stabilized reference or calibration system and, more particularly, to a method and system for stabilizing the concentration of a dissolved gaseous species of interest in a calibration sample medium by the provision of a reversible compensating reservoir or independent temperature dependent source of the dissolved species of interest which compensates the temperature dependency in the sample medium over the useful temperature range of the system.
II. Description of the Related Art
The field of diagnostic medicine is fast becoming more sophisticated and complex. The ability to make rapid or immediate diagnostic determinations characteristic of the current condition of a patient so that the proper emergency steps may be taken in a timely manner to improve or stabilize the condition of the patient, for example, during surgery or during the treatment of traumatic injury has become very important. The partial pressures of oxygen (PO.sub.2) and carbon dioxide (PCO.sub.2) in the blood are examples of extremely important instantaneous indications of respiratory deficiency, efficiency of inhalation therapy, renal function and other vital bodily processes. These measurements are made utilizing stationary clinical laboratory instruments which measure the parameters at a specific temperature and which are periodically calibrated to operate by a calibration system which is accurate only at a specific temperature. Calibration and use of the instruments are thus keyed to opening the calibration fluid, calibrating the instrument, and operating the instrument at a specific known temperature, e.g., 37.degree. C. The specific composition of typical control or calibration fluid systems is such that the reference or known equilibrium partial pressures of oxygen and carbon dioxide are temperature dependent and so occur only at the specific calibration temperature, and opening or using the calibration system at a temperature other than the designed temperature may introduce a decided amount of error into the readings. Alternatively, gaseous CO.sub.2 and O.sub.2 can be used to calibrate the instrument, but that requires the need for compressed gas cylinders for storage of the gases.
A definite need exists for a calibration system which is small, easily portable and less temperature dependent to thereby enable the calibration of such devices as diagnostic systems of the class described to be accurately accomplished over a reasonable range of ambient temperatures. This, of course, applies to the temperature stabilization of the amount of contained dissolved or dissociated gaseous species of interest in a variety of media for a variety of applications. In this regard, particularly with respect to oxygen, one approach that has been used involves the use of relatively inert fluids which have the ability to dissolve rather large amounts of oxygen and which are stable with respect to biological media. One class of such compounds consists of fluorinated organic compounds known as perfluorocarbons. Certain other systems or approaches have been proposed to stabilize and buffer the partial pressure of species such as carbon dioxide, normally in an aqueous phase, which, when combined, may be emulsified with the perfluorocarbon phase.
In a co-pending application Ser. No. 07/604,666, filed Oct. 26, 1990 and assigned to the same assignee as the present invention, one approach is disclosed in which a perfluorocarbon-based system is used which contains sufficient perfluorocarbon to give it ability to remain stable with respect to the relative concentration of dissolved oxygen over a range of ambient temperatures for example, between about 20.degree. C. and 30.degree. C. An aqueous solution phase, which contains a specific amount of one or more CO.sub.2 complexing agents, such as ethylene diamine, HCO.sub.3.sup.-, Ca++ and OH.sup.-, or the like, as agents to buffer the partial pressure of carbon dioxide in the aqueous phase is used to reduce sensitivity to changes in temperature with respect to complexed CO.sub.2. To the extent any information is deemed necessary to the proper completion or explanation with regard to material contained in the present application, such may be deemed incorporated by reference herein.
The present invention involves a simple and accurate method for reversibly controlling the partial pressure of a contained dissolved or dissociated gaseous species of interest in a medium which exhibits time/temperature stability that would be applicable to a species such as carbon dioxide. Thus, for example, the design of a pre-packaged calibration system which is usable to quickly and accurately calibrate an instrument over a range of ambient temperatures is now possible.